Wave filter



Feb. 12, 1952 w, D. LEWIS ETAL WAVE FILTER Filed Sept. 17, 1949 F/G. 4 Hw 3900 3920 SNO 3960 39.0 000 (020 FREOUENCV-MEGCYCLES WD. EW/5 W. W. MUA/FORD 'ATTORNEY Patented Feb. 12, 1952 -UNITED STATES PATENT OFFlCE WAVE FILTER Application September 17, 1949, Serial N0. 116,342

Claims. (Cl. 118-44) The application of William W. Mumford, Serial No. 24,257, led April 30, 1948, now Patent No. 2,540,488, discloses a microwave filter comprising a plurality of resonant chambers connected in tandem by interposed sections of wave guide. Each chamber is formed by a pair of susceptive discontinuities located at the respective ends of a section of wave guide. The iilter is of the maximally-iiat type, that is, it has a substantially constant insertion loss and a reiiection coeiiicient which is substantially zero over as wide a band of frequencies as possible. In order to provide such a characteristic the-band widths passed by the different chambers increase progressively from the center to both ends of the iilter. This tapering of the band widths is accomplished by controlling the susceptances of the different pairs of discontinuities which form the resonant chambers. If the iilter has N resonant chambers it will thus require 2N discontinuities and N-l connecting sections of line.

The microwave filter in accordance with the present invention is the electrical equivalent of the maximally-dat prototype lter just described but requires only N+1 susceptive discontinuities and is considerably shorter in length. The derivation of the new structure is based upon the equivalence of a section of wave guide with a single centrally-positioned discontinuity to a connecting sectionfwith half of a resonant chamber, including a discontinuity, at each end thereof.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings in which like reference characters designate similar or corresponding parts and of which:

Fig. l is a longitudinal sectional view of the prototype microwave iilter comprising resonant chambers connected by sections of wave guide;

Fig. 2 is a cross-section of the filter of Fig. 1 taken at the line 2--2;

Fig. 3 is a schematic circuit of a portion of the filter of Fig. 1 comprising a connecting section with half of a resonant chamber at each end thereof;

Fig. 4 is a schematic circuit of a simplified lter section, equivalent to the one shown in Fig. 3,

2 comprising a section of wave guide with a single susceptance at the center;

Fig. 5 is a longitudinal sectional View of a microwave filter in accordance with the invention comprising a plurality of sections of the type represented by Fig. 4;

Fig. 6 is a cross-sectional view of the filter of Fig. 5 taken at the line 6 6; and

Fig. '7 gives the theoretical and measured insertion loss-frequency characteristics of the lter of Figs. 5 and 6. v

Taking up the iigures in greater detail, Figs. 1 and 2 show the prototype maximally-flat microwave filter of the type disclosed in the above-mentioned Mumford application. The iilter is symmetrical about the transverse center line I0 and comprises a rectangular wave guide II with six pairs of susceptive discontinuities longitudinally spaced therein to form six chambers, resonant at the mid-band frequency fo, and iive interconnecting sections. There are thus required twice as many discontinuities as resonant chambers. Electromagnetic wave energy from a suitable source is fed in .at one end, as indicated by the arrow I2, and the iiltered energy is extracted at the other end, as indicated by the arrow I3, and utilized in an appropriate load device.

Each of the discontinuities is in the form of an inductive iris formed by a transverse partition I4 with a central aperture II extending between the wider sides I5, I6 of the guide in the direction of the electric iield E, as shown in Fig. 2. The susceptance of the iris is determined by the width of the aperture I'I in a direction perpendicular to the vector E. There are six pairs of these irises II, I8, I9, I9', I8 and I'I' forming six resonant chambers 20, 2l, 22, 22', 2l and 20 connected by the iive sections 23, 24, 25, 24 and 23. Each of the resonant chambers has a length approximately equal to half a wavelength Ag within the guide il and each con necting section has a length approximately equal to three quarters Ag. All of the chambers are resonant at ,fo but their band widths increase from the center of the filter to each end to provide a maximally-flat characteristic. This is accomplished by increasing the width of the apertures, and thus decreasing the susceptance of the different pairs of irises, as the ends of the filter are approached from the center. Thus,

the iris I8 is larger than I9, and il larger than I Fig. 1 by replacing a number of two-iris sections by equivalent one-iris sections. To see how this is done, consider the filter of Fig. 1 divided into seven sections 30, 3l, 32, 33, 32', 3|' and 30 by transverse planes through the centers of the resonant chambers 20, 2l, 22, 22', 2l and 20', respectively. The end sections 30 and 3D' are used, unchanged, as the end sections 40 and 40 of the filter of Fig. 5. Each comprises a section of wave guide of length Si, equal to half of the length of the chamber 2B, and an iris having a normalized susceptance Bi connected at the outer end thereof.

Each of the intermediate sections ln Fig. l consists of a connecting line with half of a resonant chamber at each end thereof. rllhus, the section 3|, as shown schematically in Fig. 3, is made up of a central section of line of length D equal to the length of the connecting guide 23, a normalized susceptance Bi at the left end thereof, a normalized susceptanceBx equal to that of the iris I8 at the right endthereof, a section of line of length Si to the left of B1 and a section of line of length SX, equal to half the length of the chamber 2|, at the right of Bx. In accordance with the invention, the lter section shown in Fig. 3 is replaced by the equivalent one shown in Fig. 4 comprising a line of length 2S2 with a normalized susceptance Bz at the center thereof. Two of these sections appear in Fig. 5, next to the end sections, as sections 4I and 4 I In like manner the sections 32, 33 and 32 in Fig. l are replaced in Fig. 5 by the equivalent sections 42, 43 and 42' which comprise sections of guide of length 2S3, 254 and 2S3, respectively, with centrally positioned irises having normalized susceptances B3, B4 and B3, respectively. When the seven sections are connected in tandem, as shown, there are thus formed the six chambers designated 5B, 5i, 52, 52', 5l and 50'. Due to the symmetry of the structure about the central susceptance B4, the chambers 5D, 5l and 52 are identical to the chambers 50', 5 l and 52', respectively.

Between each two adjacent rises there is provided a tuning screw 45, with lock nut 46, for adjusting the chamber defined by the irises and the connecting guide.

It will be noted that the lter of Fig. 5 requires only seven rises to provide a structure which is the electrical equivalent of the prototype filter with twelve rises shown in Fig. l. Furthermore, the former is considerably shorter in length than the latter since the siX chambers are directly coupled together and do not require connecting sections of wave guide.

Design formulas for evaluating the susceptances of the rises and the lengths of wave guide for the different sections of a directly-coupled filter of the type shown in Fig. 5, to make it the lectrical equivalent of the prototype maximally-fiat lter of Fig. l, will now be presented. In the general case there will be N chambers (in Fig. 5 N=G) formed by N-i-l lter sections each comprising a length of wave guide and an iris. The lter will thus comprise two end sections of wave guide each of length Sm, one or more intermediate sections of wave guide each of length 2Sm, an iris having a normalized susceptance B1 connected at the outer end of each of the end sections, and an iris having a normalized susceptance Bm connected at the center of each of the intermediate sections. The subscript m denotes the number of the section counted from the nearer end of the lter, and in Fig. 5 is one for the end sections 40 and 40', two for the next sections 4I and 4I', three for the sections 42 and .i where N is one less than the total number of sections, Ao is the mid-band free space wavelength, )w is the mid-band wavelength in the guide, fo is the mid-band frequency, and if is the band width to the three-decibel insertion loss points. The vertical lines on each side oi Bm indicate that its absolute value is given by Equation 2.

The solid-line curve of Figure 7 shows the insertion loss in decibels plotted against the frequency in megacycles for a seven-section (sixchamber) microwave filter of the type shown in Fig. 5 having a mid-band frequency fn oi 3970 megacycles and a band width Af of 8O megacycles built as a unit in a rectangular brass wave guide having internal dimensions of 1.872 inches by 0.872 inch. The chambers were first adjusted individually for minimum insertion loss by means of the tuning screws 45. A wave-guide bridge and a scanning oscillator were then used in a nal adjustment to reduce standing waves in the transmission band.

For comparison, the broken-line curve of Fig. 7 shows a calculated theoretical characteristic which neglects dissipation and therefore shows lower insertion loss in the band. Otherwise, the measured curve is in good agreement with the calculated curve. The agreement between the two curves becomes closer as the ratio of the band Width Af to the mid-band frequency jo decreases.

Although Fig. 5 shows for illustrative purposes a filter having live intermediate sections it is to be understood that either more or less may be used. The invention is of particular importance in designing highly discriminative filters with four or more intermediate sections.

What is claimed is:

l. A wave iilter comprising two end sections of wave guide each of length Sm, at least four intermediate sections of wave guide each of length 25m connected in tandem therewith, an iris having a normalized susceptance Bi connected at the outer end of each of said end sections, and an iris having a normalized susceptance Bm connected at the center of each of said intermediate sections, where Sm-47V arc cot -2- N is one less than the total number of said sections, the subscript m denotes the number of the associated section counted from the nearer end of the filter, M is the mid-band free space wave* length, Ag is the mid-band wavelength in the sections of guide, fn is the mid-band frequency, and

Af is the width of the transmission band to the f three-decibel insertion loss points, each of said sections of wave guide comprising a hollow conductor adapted to convey therethrough electromagnetic microwave energy above a critical frequency.

2. A filter in accordance with claim 1 which includes tuning means associated with each of said sections.

3. A iilter in accordance with claim 2 in which 6 said tuning means comprise a screw extending through a Wall of the associated section of wave guide.

4. A lter in accordance with claim 1 in which said sections of wave guide are made of brass.

5. A iilter in accordance with claim l in which the number of said intermediate sections is at least five.

WILLARD D. LEWIS. WILLIAM W. MUMFORD.

No references cited. 

